Attrition of the Charge on a Carrier Moving Through a Gas

Metallic, free-flying probes were used for sensing areas filled with weak concentration of free, unbalanced ambipolar charge or plasma, or for determining the charge on aerosols. Depending on the conditions, problems can be encountered in the behavior of the probe. This paper focuses on the charge exchange processes occurring between a small metal charge carrier and the gaseous medium through which the carrier travels; these processes may lead to a charge imbalance. In the experiments described, a small lead pellet was propelled with a small volume of pressurized carbon dioxide on a horizontal ballistic trajectory in air. At the launch site, the probe was charged by triboelectrification in a polytetrafluoroethylene tube, and its pre-and post flight charge measured with dynamic Faraday cups. The charge loss, incurred by the pellet crossing a 96 cm path (40 ms flight time), was 11%. Analysis of the results suggests that the onset of discharge from the carrier's body to the gas medium is a likely cause for the observed results.